Liquid phase oxidation



Patented May 8, 1951 LIQUID PHASE OXIDATION Carroll A. Hochwalt, Dayton, Ohio, assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware" N Drawing. Application March 22, 1949, Serial No. 82,913

This invention relates to liquid phase oxidation of alkyl substituted aromatic compounds in the presence of catalysts. More specifically the invention relates to a method of promoting the catalytic oxidation by conducting it in the presence of certain substances which provide free radicals.

The liquid phase oxidation of alkylbenzenes, for example xylene, in the presence of metal oxide or metal salt catalysts is well known to the art. These oxidations have been quite useful in the preparation of partially oxidized products, for example toluic acids, but only very small proportions of the dibasic acids have heretofore been prepared.

The primary purpose of this invention is to provide an improved methodwhereby substantial yields of acids may be prepared by the oxidation of alkylbenzenes. A further purpose of this invention is to provide new and practicable methods for preparing both monobasic and dibasic acids by oxidation of dialkylbenzenes with gaseous oxygen. A still further purpose is to increase the yield of terephthalic acid in the oxidation of p-xylene.

Liquid phase oxidations oi alkylbenzenes are conventionally conducted in closed retorts under pressure, if necessary, to retain the hydrocarbon at the temperature of reaction. Pressures from 50 to 1000 pounds per square inch are useful, and temperatures of 100 to 250 C. have been used. It is also known that a wide variety of metal oxides and salts arevery effective in accelerating the oxidation, for example cobalt oxide, lead oxide, barium oxide, iron oxide, manganese oxide, vanadium oxide, chromium oxide, copper oxide, nickel oxide or the corresponding hydroxides or salts of the same metals or mixtures of them. Under conditions of oxidation the salts and some of the oxides are probably converted to salts of the carboxylic acids present in the reaction mass and the catalytic effect is achieved by the metal in this form. The oxidations are usually conducted by means of oxygen, air, oxygen enriched air, or any other mixture of oxygen and inert gas, in corrosion resistant reactors, for example nickel, stainless steel or glass-lined steel. When oxygen is used in high concentrations there is some explosion hazard; and therefore preferred use involves mixtures of oxygen and inert gases wherein the oxygen is less than fifty percent by volume.

The oxidation of alkylbenzene can be prac- 12 Claims. (Cl. 260-524) ticed with any compound having a benzene nucleus and at least one alkyl substituent, for

example toluene, o-xylene, m-xylene, p-xylene, mixed xylenes, ethylbenzene, the various isometric diethylbenzenes, and the hydrocarbons having one or more other alkyl substituents, such as n-propyLisopropyl andthe different butyl radicals, as for example in cumene and cymene. Of particular importance are the dialkylbenzenes wherein the alkyl radicals are the same or different and each alkyl radical has from one to four carbon atoms.

It has been found that the chemical action is promoted by the presence of certain substances which yield free radicals under the conditions of the reaction. Not all substances known to be productive of free radicals are effective in inducing the promotion efiect, but the critical characteristics of promoters have not yet been definitely ascertained. Accordingly, predictions of the operativeness of various free radical producing substances are not possible. In accordance with this invention it has been found that substantial increases in yield of both monobasic and dibasic acids are obtained when the reaction is conducted in the presence of a small proportion of tetraalkyl leads, for example tetraethyl lead. In the practice of this invention it has been found desirable to use from 0.05 to 2.0 percent by weight of tetraethyl lead based upon the weight of the alkylbenzene charged, although larger and smaller amounts may be used.

It has been found that the reaction is autocatalytic and that sometimes a period of induction is required before rapid oxidation begins. The induction period is especially noticeable when a new reactor is used, or when an old reactor is used after not being used for a period of time, or after being used for a different reaction. It

is believed that traces of product on the surface of previously used reactors catalyzes the reaction and eliminates or minimizes the usual period of induction. The addition of a small quantity of an oxidation product, for example monocarboxylic acid or a dicarboxylic acid corresponding to the alkylbenzene being oxidized, is beneficial. In the oxidation of p-xylene to terephthalic acid a small quantity of p-toluic acid may be used to reduce the length of the induction period.

Further details of the practice of this invention are set forth with respect to the following specific examples.

Example 1 amechanical'agitator consisting of a three blade propeller type and an electric motor suitable for driving the propeller at 1,750 revolutions per minute. A gas inlet tube was so positioned so as to be immersed in the reaction mass at a point just above the propeller. The autoclave was vented through a water cooled condenser and then through a valve to regulate the flow of exit gas. The condenser was provided with a means for separating the unreacted xylene and returning it to the autoclave. The temperature was maintained automatically by means of an electric heater and a thermocouple. The flow of gas through the autoclave was maintained by means of an air stream at a constant pressure and regulated by means of the'valve in the vent line.

The autoclave was charged with 500 grams of p-xylene, 1.5 grams of cobaltous hydrate and 2.5 grams of p-toluic acid. The temperature was maintained at 160 to 170 C. for four hours, during which time the reactor was maintained at 200 pounds per square inch and air was provid'ed 'at the rate of cubic feet per hour. The analysis of the product showed a 10.3 percent conversion to terephthalic acid.

Example 2 Usingthe procedure identical'to that of the preceding example except that the autoclave was also charged with 1.0 gram of tetraethyl lead. The terephthalic acid conversion was thereby increased .-to 16.7 percent.

Example 3 The procedure of Example 1, was duplicated except that m-xylene was used in place of .pxylene. A 4.2 percent yield of isophthalic acid was recovered. .An identical .run was made in which 1.0.gram of tetraethyl lead was also charged to the autoclave, whereby 8.3 percent of isophthalic acid was obtained.

:Incopending application Serial No. 82,921 filed March '22, 1949 by William S. Emerson and Robert A. 'I-leimsch, there are described and claimed methods of conducting .the liquid phase 1 oxidation in two phases of .varying conditions of operation. The .first stage of the reaction is conductedat lower temperaturesand the second stage, if substantial oxidation has taken place,.is conducted at higher temperatures. -If desired the initial stage may be conducted with a substantially lower oxygen supply which is increased as .the reaction approaches completion. The combination of one or both of these 'expedients withithe use of tetraalkyl lead, particularly tetraethyl lead, will induce much better yields than are described in the above examples.

Theiinvention is defined by the following claims.

Iclaim:

1. The catalytic liquid phaseoxidation of dialkylbenzenes to dicarboxylic acids, which comprises contacting a dialkylbenzene having from oneto four .carbon atoms in-thealkylgroups with an Oxy en containing gas in the presence of an oxidation catalyst of the group consisting of metal oxides, .metalsalts, and mixtures thereof at a temperature in excess of 140 0., in the presence oftetraalkyl lead.

2. The catalytic liquid phase oxidation of di alkylbenzenes to-dicarboxylic acids, which comprises contacting a dialkylbenzene having from one to four carbon atoms in the alkyl groups with an oxygen containing gas in the presence of an oxidation catalyst of the group consisting ofmetal oxides, metal salts, and mixtures thereof at a temperature in excess of C., in the presence of tetraalkyl lead.

3. The catalytic liquid phase oxidation of dialkylbenzenes to dicarboxylic acids, which comprises contacting a dialkylbenzene having from one to four carbon atoms in the allryl groups with an oxygen containing gas in the presence of an oxidation catalyst of the group consisting of metal oxides, metal salts, and mixtures thereof at a temperature between 140 C. and 200 C. in the presence of tetraalkyl lead.

4. The catalytic liquid phase oxidation of dialkylbenzenes to dicarboxylic acids, which comprises contacting a dialkylbenzene having from .one.to four carbon atoms in the alkyl groups with an oxygen containing gas in the presence of an oxidation'cata'lyst of the group consisting of metal oxides, metal salts, and mixtures thereof at a temperature between 140 C. and 200 C., in the presence of tetraethyl lead.

5. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises heating a xylene with an oxygen containing gas in the presence of a cobalt containing catalyst in the presence of from 0.05 to 2.0 percent by weight of-tetraalkyl lead.

6. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises heating a xylene with an oxygen containing gas in'the presence of a cobalt containingcatalystand from 005 to 2.0 percent by weight of tetraethyl lead.

"7. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises contacting a xylene with an oxygen containing gas in the presence of a cobalt containing catalyst and from 0.05 to 2.0 percent by weight of tetraalkyl lead, at a temperature in excess of 140 C.

8. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises contacting a xylene with an oxygen containing gas in the presence of a cobalt containing catalyst and from 0.05 to 2.0 percent by weight of tetraethyl lead, at a temperature in excess of 140 C.

9. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises contacting a xylene with an oxygen containing gas in the presence of a cobalt containing catalyst at a temperature between 140 C. .and 200 C., in the presence of from 0.05 tc.2.0 percent by weight of tetraalkyl lead.

'10. The catalytic liquid phase oxidation of a xylene to a dicarboxylic acid, which comprises contacting a xylene with an oxygen containing gas in the presence of a cobalt containing catalyst at a temperature between 140 C. and 200 0., in the presence of from 0.05 to 2.0 percent by weight. oftetraethyl lead.

11'. The catalytic liquid phase oxidation of pxylene to terephthalic acid, which comprises contacting p-xylene with air at a temperature between 140 and 200 C. in the presence of from 0.05 to 2.0'percent by weight of a tetraalkyl lead anda cobalt containing catalyst.

12. The catalytic liquid phase oxidation of p xylene to terephthalic acid, which comprises contacting p-xylene with air at a temperature between 140 and 200 C., in the presence of from 0.05 to 2.0 percent by weight of 2, tetraethyl lead anda cobalt containing catalyst.

CARROLL A. HOCHWALT.

(References on following page) REFERENCES CITED FOREIGN PATENTS The following references are of record in the Number Country Date file of this patent: 364,442 Germany Nov. 25, 1922 UNITED STATES PATENTS 5 OTHER REFERENCES Number Name Date Berezovskaya. et aL, Chem. Abstracts, col. 2024 1,939,255 Egloff Dec. 12, 1933 (1945). 2,120,672 Mares June 14, 1938 2,215,472 King et a1 Sept. 24, 1940 2,245,528 Loder June 10, 1941 10 2,276,774 Henke et a1 Mar. 17, 1942 2,479,067 Gresham Aug. 16, 1949 

1. THE CATALYTIC LIQUID PHASE OXIDATION OF DIALKYLBENZENES TO DICARBOXYLIC ACIDS, WHICH COMPRISES CONTACTING A DIALKYLBENZENE HAVING FROM ONE TO FOUR CARBON ATOMS IN THE ALKYL GROUPS WITH AN OXYGEN CONTAINING GAS IN THE PRESENCE OF AN OXIDATION CATALYST OF THE GROUP CONSISTING OF METAL OXIDES, METAL SALTS, AND MIXTURES THEREOF AT A TEMPERATURE IN EXCESS OF 140* C., IN THE PRESENCE OF TETRAALKYL LEAD. 